A piston-cylinder device contains 0.85kg of refrigerant at -10 deg C. The piston that is free to move has a mass of 12kg and diameter of 25cm. the local atmospheric pressure is 88kPa. Now, heat is transferred to the refrigerant-134a until the temperature is 15 deg C. Determine the final pressure. The solution in the textbook says that the final pressure is equal to the initial pressure. I don't understand how this is an isobaric process.